Some diesel engines have a so-called group hole nozzle (GHN) configured to include a plurality of injection hole groups having a plurality of injection holes for injecting fuel, such that fuel injected by each of the plurality of injection holes will form a single fuel spray cloud by each group, and thereby reduce a radius of each injection hole and atomize fuel while attaining a sufficient total flow cross sectional area of the injection holes by increasing the number of injection holes.
One example of this type of diesel engine is described by U.S. Pat. No. 7,201,334. This reference describes addressing soot (black exhaust) reduction due to enhancement of fuel atomization and strengthening fuel spray penetration by devising an angle between axes of injection holes in each injection hole group.
Using GHN technology, such as the technology described in U.S. Pat. No. 7,201,334 and enhancing fuel atomization can be useful for reducing soot emitted from a diesel engine. However, in some cases engine components such as fuel injection nozzles, combustion chambers, etc., are configured such that a fuel is ignited after the fuel collides with a wall surface of a combustion chamber to increase ignition lag of the injected fuel. In such a case, it is also important to facilitate reheating due to mixing combusted gas and surplus air by strengthening a vertical vortex in the combustion chamber, and to enhance fuel atomization to reduce soot even further, and/or to reduce nitrogen oxide (NOx) sufficiently in addition to reduction of soot.
To strengthen a vertical vortex in the combustion chamber, the penetration force of fuel spray after the fuel collides with a wall surface of a combustion chamber can be increased, which can in turn enhance swirl and penetration longitudinally along the wall surface of fuel spray and combusted gas downstream of a combustion zone, in addition to increasing a penetration force of fuel spray before the fuel reaches the wall surface.
Fuel spray injected into a combustion chamber of a diesel engine may collide with a wall surface of a cavity provided on the top portion of a piston during an ignition lag period and may spread along a wall surface of the cavity by setting the fuel spray penetration properly.
The fuel spray, then, combusts most efficiently near the wall surface, and combustion gas (burned gas) and fuel spray are carried about by a vertical vortex stream induced by a combustion expansion flow, and swirl and penetrate longitudinally along the wall surface.
When the mixture of fuel spray and burned gas swirling and penetrating around the wall surface rapidly reach the center of the cavity, high-temperature burned gas is cooled rapidly by mixing with low-temperature surplus air since there is low-temperature surplus air including plenty of oxygen not used for combustion around the center portion of the cavity. This can result in a decrease in NOx production and a reduction in soot by contacting soot included in burned gas with oxygen and reheating it.
Therefore, by increasing the penetration force of the fuel spray after the fuel spray collides with the wall surface, and by enhancing swirling and penetrating around the wall surface of fuel spray and combusted gas, burned gas can mix with surplus air rapidly, thereby reducing NOx and reheating soot to reduce soot in emissions.
However, the reference described above is designed to maintain spray penetration force by colliding atomized fuel sprays with each other and utilize all air in the combustion chamber space from the injection hole to the combustion chamber wall surface, and thereby complete combustion substantially before the fuel spray reaches the wall surface of the combustion chamber.
So, this reference does not consider enhancement of fuel spray penetration after the fuel spray collides with the wall surface, and therefore it can not enhance penetration force of the fuel spray after the fuel spray collides with the wall surface to reduce generation of NOx and soot sufficiently.
Therefore, there is a need for providing a diesel engine that can enhance penetration force of fuel spray formed from fuel injected into a combustion chamber of engine cylinder after the fuel spay collides with a wall surface of the combustion chamber, to reduce generation of NOx and soot sufficiently.
According to a first aspect of an embodiment of the present description, a diesel engine is disclosed, which comprises a cavity provided on a top surface of a piston of said engine, the cavity having a concave cross section along a moving direction of said piston, and forming a combustion chamber. The engine further may include a fuel injection nozzle located such that the fuel nozzle is facing a substantially center portion of said combustion chamber and is configured to inject fuel to a side wall of said combustion chamber. The concave cross section may have a shape in which a center of a bottom portion is raised up toward an opening of said concave cross section, the center being located along a radial direction of said piston. The fuel injection nozzle may have a plurality of injection hole groups, each group having two injection holes respectively. A distance between said two injection holes and an angle between longitudinal axes of said two injection holes of each of said injection hole groups may be each set such that fuel sprays injected from said two injection holes will form a single fuel spray cloud for each of the injection hole groups after the fuel sprays collide with a wall of said combustion chamber, and such that the distance between collision points of the fuel sprays injected from said two injection holes at a time of their collision with said wall of said combustion chamber will be in a predetermined range in which a penetration force of said fuel spray cloud along a longitudinal direction of said combustion chamber received after collision with said wall of said combustion chamber is at or near a predetermined maximum value.
This diesel engine overcomes at least some of the disadvantages of the approach of the related reference described above.
In one example embodiment, the predetermined range is a range in which said penetration force of said fuel spray cloud along the longitudinal direction of said combustion chamber will be 120% or more as large as a penetration force of said fuel spray cloud along a lateral direction of said combustion chamber.
According to a second aspect of the embodiment of present description, a diesel engine is provided, which comprises a cavity provided on a top surface of a piston of said engine, the top surface having a concave cross section along a moving direction of said piston, and forming a combustion chamber. The engine may further comprise a fuel injection nozzle located such that the fuel nozzle is facing a substantially center portion of said combustion chamber is configured to inject fuel to a side wall of said combustion chamber. The concave cross section may have a shape in which a center of a bottom portion is raised up toward an opening of said concave cross section, the center being located along a radial direction of said piston. The fuel injection nozzle may have a plurality of injection hole groups, each group having two injection holes respectively. A distance between said two injection holes and an angle between longitudinal axes of two injection holes of each of said injection hole groups maybe each set such that fuel sprays injected from said two injection holes will form single fuel spray cloud for each of the injection hole groups after the fuel sprays collide with a wall of said combustion chamber, and such that a distance between collision points of the fuel sprays injected from said two injection holes at a time of their collision with said wall of said combustion chamber will be in a range from 4.5 to 7.5 millimeters.
This diesel engine also overcomes at least some of the disadvantages of the approach of the related reference described above.
In another example embodiment, the distance between respective centers of an outlet of each of said two injection holes in the plane along the moving direction of said piston is in a range from 0.25 to 0.5 millimeters.
In another example embodiment, the distance between respective centers of an outlet of each of said two injection holes in the plane perpendicular to the moving direction of said piston is in a range from 0.25 to 0.5 millimeters.
In another example embodiment, the angle between the respective longitudinal axes of the two injection holes in the plane perpendicular to the moving direction of said piston is in a range from 7.5 to 12.5 degrees.
In another example embodiment, the angle between the respective longitudinal axes of the two injection holes in the plane perpendicular to the moving direction of said piston is in a range from 7.5 to 12.5 degrees.
In this way, at least some of the disadvantages of the related reference described above are overcome.